Magnetochemistry: advances in theory and experimentation

We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.

SQUID magnetometers for low-frequency applications

The operation and noise limitations of de and RF SQUID's are outlined, and recent advances in their sensitivity are discussed. A model of the dc SQUID predicts an energy noise level per hertz referred to the SQUID of approximately 8 k_{B}TL/R \approx 8 k_{B}T(\piLC)^{1/2} , where L, R, and C are the SQUID inductance and the shunt resistance and capacitance of each Josephson junction. Some examples of dc SQUID's are described to show that their performance is generally in reasonable agreement with the model. The noise energy has improved from about 2 × 10-30J. Hz-1for a device with L = 1 nH and a tunnel junction area of 104µm2to about 2 × 10-33J . Hz-1for a device with L = 0.1 nH and a microbridge resistance of 40 Ω. Further improvements axe expected in the near future. The model of the RF SQUID predicts a noise energy per hertz referred to the SQUID of [(\pi\alpha^{2}\phi\min{0}\max{2}/2L) + 2 \pi \alpha k_{B}T\min{a}\max{(eff)}]/\omega_{RF} , where α is the intrinsic width of the distribution of flux transitions, T\min{a}\max{(eff)} is an effective amplifier noise temperature, and ω RF is the pump frequency. With one exception, the performance of the seven types of RF SQUID listed is in reasonable agreement with the model. The noise energy ranges from about 1.5 × 10-29J . Hz-1for a 20-MHz toroidal SQUID to 3.5 × 10-31J . Hz-1for 9-GHz reentrant toroidal SQUID; a somewhat better sensitivity has been reported for a 430-MHz device, apparently in conflict with the theory. In both dc and RF SQUID's, 1/ f noise ( f is frequency) is likely to extend to higher frequencies as the white-noise level is decreased.

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Advances in SQUID magnetometers

The problem of coherent flux tunnelling across a Josephson weak link enclosed by a thick superconducting (‘SQUID‘) ring in terms of quantum electrodynamics is considered. We show that for a low effective noise temperature (≃few K) in the SQUID ring and with the weak link capacitance ≲3 × 10−15, such a process should be observable. Experimental evidence is given to justify this claim.

Macroscopic quantum electrodynamic effects in a superconducting ring containing a Josephson weak link

We discuss the response of a superconducting weak-link ring to externally applied adiabatic noise and show that this can be used as a probe of the low-lying energy eigenstates of the ring. We compare the observable consequences of this with new experimental results.

The response of a superconducting weak-link ring to external adiabatic noise

We describe a 430‐MHz SQUID magnetometer in which both the resonant frequency of the SQUID tank circuit and the input/output impedances of the following 4.2 K GaAs FET amplifier are varactor controlled. The intrinsic energy sensitivity of the magnetometer, in full flux lock from dc to 5 MHz, is 6.8×10−32 J/Hz (≡4×10−6Φ0/√Hz flux sensitivity). The figure of merit is 2.1×10−31 J/Hz.

R. J. Prance

Papers

Varactor tuned ultrahigh frequency SQUID magnetometer